Silver halide emulsion materials are particularly useful in the production of black-and-white and colour images with high optical density and high resolving power but require in conventional processing aqueous alkaline processing liquids that may not come into contact with the skin because they have an irritating and skin destroying effect. Moreover, alkaline aqueous solutions pose ageing problems in that they become gradually neutralized by carbon dioxide absorbed from the air.
Under the impulse of said specific drawbacks and of ecological requirements there has been looked for a process wherein the alkaline substance is formed in situ during the processing of the photographic materials by means of originally non-corrosive alkali-generating chemicals and an aqueous liquid the pH of which is not much above 7 or wherein simply neutral plain water is used.
In the well known diffusion transfer reversal (DTR-) processing [ref. e.g. Photography--Its Materials and Processes--by C. B. Neblette--16th ed. D. Van Nostrand Company--New York (1962), p. 372] an exposed silver halide emulsion material is developed in alkaline medium in the presence of a silver ion complexing agent, also called silver halide solvent. Hereby the non-developed silver halide is complexed and transferred by diffusion into an image-receiving material to form therein a silver image by reduction with the aid of a developing agent in the presence of minute amounts of so-called development nuclei, e.g. colloidal silver or heavy metal sulphides. More details about the DTR process and substances used therein are given by Andre Rott and Edith Weyde in their book: "Photographic Silver Halide Diffusion Processes"--Focal Press--London, New York (1972).
Common in DTR-processing is the use of photosensitive silver halide emulsion materials that contain the necessary developing agent(s) applied already at their coating stage. The processing of such materials proceeds with a so-called activator solution which is a purily alkaline aqueous solution having originally a pH between 12 and 13 (see the above book of Andre Rott and Edith Weyde, p. 81).
In U.S. Pat. No. 3,260,598 a process for forming a silver negative image and for forming simultaneously a silver positive image in a processing element is described wherein for alkali-release in situ a very slightly watersoluble metal hydroxide Z(OH).sub.n is allowed to react with a compound XY, wherein in the hydroxide Z represents a metal atom selected from the class consisting of cadmium, aluminium, zinc, titanium and lead, and in the XY-compound X represents sodium or potassium and Y represents a citrate radical, ferrocyanide radical, fluoride ion, tartrate radical, an ethylenedinitrilo tetraacetate (EDTA) radical, a 1,3-diamino-2-propanol tetraacetate radical, a trimethylaminetricarboxylate radical, a di-.omega., .omega.-methylaminodiethylaminedicarboxylate radical or a di-.omega., .omega.-methylaminodiethylaminetetracarboxylate radical.
For ecological reasons most of the above mentioned metal atoms represented by Z may not be introduced in the draining waste water or only in very limited concentrations. Moreover, as mentioned in published EP-A 0210659 the alkali-generation with said system has been found to be not very efficient.
In said published EP-A 0210659 a process for generating alkali is described, wherein a complexing agent such as sodium picolinate is allowed to react with e.g. zinc hydroxide or basic zinc carbonate to set free hydroxyl ions whereby the pH is raised.
Picolinic acid and the picolinates are rather expensive compounds so that preference is given to more economic base generating agents that are ecologically acceptable which is not the case e.g. for phosphates and non-biodegradable ethylenediaminetetraacetic acid (EDTA).